Source

The icon must be an uncompressed, 24-bit format TGA, 16x16 pixels in size. The path is relative to the mod's installation directory (i.e where GameInfo.txt is) and should not include the TGA file extension. In the example above, the file would be "path/to/icon.tga" inside the mod directory.

Mod makers should also include a 32x32 pixel version of their mod icon. This should be in the same directory and have the same filename as the standard 16x16 icon but with "_big" appended. In the above example, the big icon would have the filename "path/to/icon_big.tga" This is used in the details view of the Steam library window. If a big icon isn't included, Steam will fall back to the 16x16 icon.

Goldsource

The icon must be an uncompressed, 24-bit format TGA, 16x16 pixels in size. The path is relative to the mod's installation directory (i.e where liblist.gam is) and should not include the TGA file extension. In the example above, the file would be "path/to/icon.tga" inside the mod directory.

Desktop icon

Creating a desktop icon for your mod is simple. Just place an icon file in the <Steam>\steam\games with the same name as the Game keyvalue specified in your GameInfo.txt/liblist.gam file. Remember to omit all colons from the icon's filename and the filename is case insensitive.

To install the icon there, use HKEY_LOCAL_MACHINE\Software\Valve\Steam\InstallPath from the registry and append \steam\games to it.